1. Field of the Invention
The field of the invention is that of methods designed to reduce the rate of false alarms in a radar receiver, especially when there is sea clutter present.
It is known that a radar receiver comprises a detector constituted by a comparator which compares the level of the received signal with a reference level known as a detection threshold. In principle, a level of signal above this threshold corresponds to the reception of an echo coming from a target.
It is also known that since the level of ambient noise is a random factor, it may happen that a signal containing only noise is strong enough to exceed the detection threshold, thus prompting what is called a false alarm. The known methods used to limit the probability of having a false alarm consist in raising the detection threshold as a function of the noise.
2. Description of the Prior Art
The problem then is to raise the threshold sufficiently so as to avoid a false alarm rate that is higher than a set rate, but not to raise this threshold excessively, in order that the echos may continue to be detected.
In particular, as regards clutter, and more particularly clutter due to the sea for onboard surface radars, it has been sought to make use of the spatially stationary state of these noises. In this type of application, it is considered that, in a given zone, defined by its elevation angle, the noise of the clutter is fairly constant. The mean value chosen may be either the maximum value of the partial mean values according to a pre-set partitioning of the distance zones, or the value according to the Kth greatest sample. According to this method, which is applicable to video signals digitally converted by sampling, it is assumed that the n-k smaller .samples contain only noise, and it is these samples that are chosen for the computation of the noise.
It has also been sought to exploit the temporally stationary state in a zone from antenna rotation to antenna rotation. It is considered in this case that, for a given zone, defined by its distance and its elevation angle, the noise can vary only slowly so that its mean may be considered as the mean value recorded on a certain number of antenna rotations.
Whether it is the stationary spatial state or the temporal spatial state that is considered, there are existing variants of implementation. Thus, with regard to the magnitude characterizing the signal, the mean value of which will be computed, we may consider:
the modulus of the signal (its amplitude) PA1 the modulus squared of the signal (its power) PA1 the logarithm of the modulus
With regard to the zones, the mean values can be computed on sliding window type zones that overlap one another partially or, on the contrary, on block type zones that exclude one another.
Finally, with regard to the adjusting of the threshold for a given zone, it is possible to take into consideration either the mean value obtained on this zone or the mean value obtained on one or more immediately adjacent zones, excluding the zone under examination.
The prior art methods that have just been described are satisfactory only inasmuch as the clutter actually has a spatially or temporally stationary state. However, measurements made by the applicant show that the assumed stationary state does not exist, especially when there are rough seas.
In rough seas, the echos obtained are not stationary in distance. There are peaks of sea clutter that appear in a distance cell as compared with the neighboring cells. Nor are they more stationary in time, for the peaks do not occur at the same positions from one antenna rotation to another. Consequently, methods of regulating false alarms that are based on temporally or spatially stationary states are not totally efficient.
One of the aims of the present invention is to provide greater efficiency to the regulating of false alarms by raising the detection threshold. It does so by proposing a method to raise this threshold by the precise quantity needed to attain a value above the value of the extreme peaks, especially when the sea is rough. The aim pursued by the invention is illustrated in FIG. 1.
FIG. 1 shows a curve A, a curve B and a curve B.sub.2.
The curve A shows a fictitious but realistic example of the noise level on a distance axis, the nearby zones being noisier than the distant zones. The curve B.sub.1 shows the value, the threshold of which has to be raised according to the criteria of stationary states of the noise.
The curve B.sub.1 is obtained by the smoothing of the curve A and the addition of a constant level value. It can be seen that this procedure leaves peaks of noise 1, 2, 3 which will give rise to a false alarm. The aim of the present invention is to replace the curve B.sub.1 by the curve B.sub.2 which, for the low levels of noise, is merged with the curve B.sub.1 and moves away from it in such a way that it increases monotonically with the level of the clutter measured on the zone. The measurement of the clutter continues to be done according to one of the known methods. The aim of the invention, therefore, is to avoid the desensitizing of the radar receiver which would result from an excessive raising of the detection threshold.
An aim of the invention is also to achieve an easy adaptation of the invention to existing radars.